The present invention relates to an improved method for preparing an oil shale deposit for in situ retorting, and more particularly it relates to a method for rubblizing and mixing rich and lean shale by the introduction of lean rubble columns in rich rubble layers to form an in situ retort in which hot fluid flow paths in the rubblized shale are maintained open under high rubble pile pressures and the presence of high retorting temperatures, both of which tend to cause plastic flow of the rich shale rubble.
The only method deemed practical by the industry for obtaining oil from oil shale is to heat the shale to the temperature at which the kerogen in the shale decomposes to yield shale oil, gas, and residue. Because oil shale is a poor conductor and is essentially impermeable before retorting, it is necessary to provide both heat transfer surfaces and space for the circulation of a hot fluid. This can best be done by introducing both fractures and space into the oil shale, a process which is referred to as rubblization. Fractures alone provide a surface but no room to circulate a hot fluid.
For surface retorts, adequate rubblization is easily achieved by mining and crushing to a suitable size. In situ rubblization is more difficult technically but has potential economic and environmental advantages. One problem is in providing fragments of suitable size with sufficient space between them to allow circulation of a hot fluid at modest pressure differences. If the fragments are too large, they will take too long to heat; if too many fine fragments are present the pressure drop required for fluid circulation will be too large for a given void space. Void space must be adequate for circulation of fluids but should be kept low to minimize costs. Another problem is the tendency of rich rubblized oil shale to undergo plastic flow under high rubble pile pressures and high retorting temperatures, thereby clogging the space between shale fragments and blocking the free flow of the hot fluid required for the extraction of oil shale products.